A radar apparatus radiates an electromagnetic wave from an antenna (antenna device), receives an echo signal from a reflecting body (e.g., target object), and detects a level of the echo signal to determine a distance and a direction from the antenna device to the target object. A radar image of the detected target object is normally displayed corresponding to the determined distance and direction on a radar screen centering on the position of the antenna device.
As the antenna devices for radar, an antenna device using a waveguide slot antenna is known (refer to JPA H04-117803). JPA H04-117803 discloses an array antenna which is configured so that a plurality of slots, each having a rectangular shape, are arranged in the waveguide. Such an array antenna realizes a radiation of the electromagnetic wave with a narrow beam width by equalizing phases of slots.
Generally, the waveguide slot antenna is often made to have an aperture distribution as the Chebyshev distribution to realize a narrow directivity beam. FIG. 1 shows the aperture distribution as the Chebyshev distribution. In FIG. 1, a side lobe level of the aperture distribution is shown as the Chebyshev distribution of −40 dB.
FIG. 2 shows the beam shape of the Chebyshev distribution shown in FIG. 1 (a radiation angle θ=90°). As shown in FIG. 2, the Chebyshev distribution has a characteristic in which the narrowest beam width is always formed at a preset constant side lobe level. Such a beam configuration is preferable for such a radar apparatus.
However, for the target object displayed on the radar screen, although there are actually various sizes of the target objects, an actual size difference of the target objects may not be reflected to the radar image. Thus, if a ratio of the sizes of two or more echo images on the radar screen is different from the actual size ratio of the actual reflecting bodies, it may possibly prevent an operator from accurately recognizing the sizes of the target objects.
For example, as shown in FIG. 3, a substantial size difference of a reflecting body (ship) 501 and a reflecting body (ship) 502 is about five times of the other. However, as shown in FIG. 4, the ships may be displayed on a radar screen only as an echo image having a 2° width and an echo image having a 5° width, respectively. That is, in this case, only about 2.5-time difference appears on the radar screen. If a substantial size difference cannot be recognized by the operator from the difference of the echo sizes between a large reflecting body and a small reflecting body on the radar screen, he/she may possibly underestimate the sizes of the target objects.
A more specific example is given and described. The following is considered assuming that the target objects shown in FIG. 3 are a ship with a displacement of 5 tons and a length of 10 m, and a ship with a displacement of 100 tons and a length of 50 m, respectively. In this case, respective radar cross-sections (RCSs) are RCS=10 m2 for the 5-ton displacement ship, and RCS=1000 m2 for the 100-ton displacement ship. Supposing that a reflection intensity of the 5-ton displacement ship is relatively about 3 dB, a reflection intensity of the 100-ton displacement ship will be about 23 dB.
Here, if the electromagnetic wave having the aperture distribution as the Chebyshev distribution shown in FIG. 1 is used, a beam width of 3 dB is approximately 2°, while a beam width of 23 dB is approximately 5°. Therefore, as shown in FIG. 2, on the radar screen, the echo image having the 2° width and the echo image having the 5° width are displayed, respectively. Thus, although the substantial size difference is about five times of the other, they appear only with about 2.5 times in difference on the radar screen, as described above.